The present invention relates to an apparatus for holding a workpiece, and more particularly to an apparatus for vacuum holding a printed circuit board substrate of any shape or size.
During the manufacture of printed circuit boards in which a printed circuit board substrate is exposed to ultraviolet light, the substrate must be securely positioned on a work surface. One type of holding apparatus secures the substrate on a work table by applying vacuum through the work table. The vacuum, however, does not always provide an effective hold, especially if the surfaces of the substrate are not perfectly flat. For example, warped circuit board substrates do not lie flat on a work table, and as a result vacuum holding ability is less than optimal. In addition, the circuit board substrates often contain through holes which prevent the vacuum holddown of the substrate.
Another common problem encountered with the use of existing holddown apparatus is that they are designed to handle a workpiece of a single preset size, and it is often necessary to process a variety of printed circuit boards substrate sizes. U.S. Pat. No. 4,468,017 describes one type of vacuum holddown apparatus that is designed to handle a variety of workpiece sizes by utilizing a vacuum zone control valve for selectively applying vacuum to certain regions on the table. One disadvantage of such a system is that the location of the vacuum source on the work table is preset and can not be moved to a location where the vacuum holding power can more efficiently operate on the workpiece.
It is therefore a principal object of the present invention to provide a vacuum holddown apparatus that will hold a printed circuit board substrate having an uneven surface.
It is another object of the present invention to provide a vacuum holddown apparatus that will hold a variety of different sizes of printed circuit board substrates.